In sputtering cathodes, the coating material present as a “target” is eroded by energetic ions from a plasma discharge, and the material set free by the ions forms, on a substrate (work piece) a coating (film). The plasma discharge is maintained in an evacuated vacuum chamber (process chamber) under controlled inlet of a working gas (inert gas) with an electric potential and a discharge current applied by a power supply between target cathode and an anode.
In the standard case of electrically conductive target materials, the target is supplied with a continuous or pulsating negative voltage, such that a dense plasma forms above the target surface. By means of the electrical field forming between plasma and target surface as cathode fall, ions from the plasma are accelerated onto the target surface, leading, because of their impact, to the erosion of the target surface and therefore to freeing of target material.
In case of electrically non-conductive target materials, the plasma discharge is maintained using a high-frequency supply leading to an ion bombardment of the target pulsating with the high-frequency, thus resulting in a sputter erosion process of the target.
In case of coatings with a composition of target materials, the material that has been set free deposits on the substrate in at least approximately the same composition as present in the target.
In case of a reactive process (reactive sputtering), additionally or alternatively to an inert gas for maintaining the plasma, one or more (additional) gases (reactive gases) are fed into the vacuum chamber with which the target material chemically reacts, such that a corresponding chemical compound forms on the substrate. Reactive processes are in particular used for oxide coatings in optical or in semiconductor industry, for forming transparent or insulating oxide coatings on the substrates.
A great advantage of sputtering processes originates from the fact that the relative material erosion off the target per process step or per process batch in case of multilayer processes is very low, and accordingly, very many processes can be carried out until the target material is used up and the target has to be replaced.
Thus, it is possible to operate a coating apparatus in the so-called load-lock mode, in which the separate substrates are fed into the process chamber via a load-lock, thereafter being coated therein, and thereafter being taken out of the system again via said load-lock. This way, the process chamber can be kept at vacuum conditions for a long time, which generally leads to a substantially improved process stability and reproducibility of the coating process.
During the lifetime of a target, certain drifts of the process conditions occur, which, e.g., in the load-lock mode, can be controlled or corrected with difficulty only. These originate from the increasing erosion of the target, from films forming on parts of the process chamber and from elsewhere. In particular, the coating thickness distribution over the extension of the substrate is subject to certain drifts during the target lifetime. Too large deviations from a target value can make it necessary to open the coating chamber several times in order to correct the relative coating thicknesses over the coating surface by means of suitable measures such as aperture corrections, geometrical corrections at the anodes or at the gas distributions, or the like. Such interruptions of a regular process cycle are not only time-consuming, but also influence disadvantageously the process stability and reproducibility, and they can require additional test processes after each correction in order to keep up the process stability and reproducibility.
In order to be able to better explain the invention, in the following, a typical setup of a known sputter magnetron (magnetron sputtering source) will be discussed. Therein, it will be referred to FIG. 1, although FIG. 1 shows an arrangement according to the invention.
In a magnetron sputtering source, the plasma density above the target is strongly increased by means of suitable magnetic fields. This is achieved mainly by means of an elongated arrangement of permanent magnets, later on also referred to as primary magnet arrangement, arranged behind the target outside the process chamber.
The arrangement of permanent magnets comprises a first part (referenced 1 in FIG. 1) and a second part (referenced 2 in FIG. 1), as schematically shown in FIG. 1. The first part 1 is formed, e.g., by magnets of one and the same field orientation arranged in a closed loop. The closed loop runs along the outer edge of the backside of the target. The second part 2 is formed, e.g., by a magnets of the opposite field orientation arranged in a row or also in a closed loop. This row or closed loop is arranged in the middle of the backside of the target.
The magnets of the first and second part can be arranged between the backside of the target and a ferromagnetic back plate 3. Through this, a mechanical as well as a magnetic connection between the first part 1 and the second part 2 of the primary magnet arrangement is provided. Magnetic field lines 12 of the magnets of the first part 1 penetrate the target 4, describe an arc shape above the target and penetrate the target 4 near the second part 2, thus forming magnetic field lines of the second part 2. Therefore, the magnetic field lines 12 form a closed-loop tunnel above the target 4, the magnetic field 12 lines running substantially parallel to the target surface 4a in the upper region of the tunnel.
Where the magnetic field lines 12 run at least approximately parallel to the target surface 4a, a particularly high plasma density 5 can form. That high plasma density 5 forms a closed region above the target surface 4a extending along a closed path approximately around the target 4. Ions from this region reach the target material via the cathode potential with particularly high energy, thus leading to the target erosion and the setting free of target material desired in the sputtering process. In case of elongated targets 4, the erosion trench formed this way reminds of a race track (cf. FIG. 2b). This closed curve of strong erosion is therefore often referred to as target erosion race track 11 or simply race track (cf. FIG. 2b). The width of this region of strong erosion is substantially influenced by the geometric shape of the magnetic field lines.
In order to achieve a high utilization of the target, it is desirable to increase the width of the region of high erosion (the width of the race track line). Since, at least usually, the permanent magnets generating the magnetron magnetic field have to be arranged behind the target and outside the process chamber, and since, in addition, a field of 10 Gauss to several 100 Gauss is required for obtaining a high erosion rate and deposition rate, the shape of the field lines 12 can be influenced to a limited amount without resulting in significant other disadvantages such as a decrease in deposition rate.
It is known that it is possible to use a secondary magnetic field (also referred to as auxiliary magnetic field) opposedly polarized with respect to the above-described magnetic field (referred to as primary magnetic field or magnetron magnetic field) for the purpose of increasing the width of the erosion region, thus increasing the utilization of the target material. The two magnetic fields (field lines referenced 12 and 16, respectively, in FIG. 1) superpose, resulting in a magnetic field (field lines referenced 17 in FIG. 1) with flatter magnetic field lines 17 near the highest point. This auxiliary magnetic field is generated by means of an additional magnet arrangement 13, also referred to as secondary magnet arrangement 13, which is arranged between the two parts 1 and 2 of the primary magnet arrangement, the polarity thereof being inverse to the polarity of the magnets of the primary magnet arrangement. This opposedly polarized auxiliary magnetic field (cf. 16 in FIG. 1) can be formed, e.g., by permanent magnets or by ferromagnetic parts (magnetic shunts or simply “shunts”) which are opposedly polarized when in the field of the primary magnet arrangement.
For example, in U.S. Pat. No. 5,262,028, permanent magnet arrangements are described which generate such an auxiliary magnetic field, thus achieving an increased target utilization.
In the arrangements described in U.S. Pat. No. 4,865,708 and in U.S. Pat. No. 4,892,633, ferromagnetic shunts as described above are arranged between the inner part and the outer part of a primary magnet arrangement as described above, which lead to the desired increase of the target utilization.
In U.S. Pat. No. 4,810,346, a different kind of magnetron sputtering arrangement is described. In this case, in addition to the primary magnet arrangement, a coil is a provided. The coil is formed by winding wire around the inner part (cf. 2 in FIG. 1) of the primary magnet arrangement. By controlling an alternating current flowing through the coil, the resulting magnetic field can be varied. It is to be noted that, due to the described arrangement of the coil relative to the primary magnet arrangement, there is substantially only one magnetic circuit formed (by primary magnet arrangement and coil together). The coil and the primary magnet arrangement are also not separate from each other, since the inner part (cf. reference 2 in FIG. 1) of the primary magnet arrangement is at the same time a part of the magnet arrangement of the coil (since the coil's wire is wound around that part). The shape of the resulting field is substantially the same as the shape of the magnetic field generated by the primary magnet arrangement alone.
Back to separate secondary magnet arrangements, it is to be noted that a secondary magnet arrangement with permanent magnets has, with respect to ferromagnetic shunts, the advantage that their magnetization is not induced by the primary (magnetron) magnetic field, which allows to provide more flexible magnet configurations, so as to allow to change the primary (magnetron) magnetic field in a more specific manner. Due to the higher magnetization, it is also possible to achieve a stronger field using a smaller magnet volume.
By means of a suitable choice of the magnitude and the precise position of such an auxiliary magnet field system (however the field may be realized), it is possible to achieve that the range in which the magnetic field lines 17 (of the field resulting from the superposition) run substantially parallel to the target surface is substantially enlarged. Since the high plasma density and, correspondingly, the high ion density forms over a wider area of the target 4, in the sputtering process a larger width of the line of the erosion race track 11 (cf. FIG. 2b) results. Consequently, more target material can be eroded until the target 4 reaches, at its thinnest place in the race track, a (usually predetermined) minimal thickness, thus having to be replaced.
In today's applications, the target utilization can readily be doubled by means of optimized secondary magnet arrangements, or even, after a suitable optimization of the magnet arrangement, be increased by a factor of 3 to 4. With such an auxiliary magnetic field functioning as an opposing field with respect to the primary (magnetron) magnetic field, the field strength above the target is decreased in the relevant area, which results in a decrease of the plasma density and, accordingly, of the ion density, which determines the rate of the sputtering process. As a consequence thereof, the voltage of the gas discharge increases, and therewith increases the electrical field strength, by means of which the ions are accelerated onto the target surface. Since the erosion efficiency of ions increases with increasing speeds, the lower ion density is to a large extent compensated for by the higher sputtering efficiency, and therefore, no substantial loss of sputtering rate results.
If a loss of sputtering rate nevertheless occurs, this can be fought also by means of other measures like, e.g., by increasing the process gas pressure or by changing the composition of the process gas, by optimizations of the geometrical configuration of the magnetron cathode, of the primary magnets.
Manufacturing applications of sputtering processes in coating apparatuses require a particularly good utilization of the coating material eroded from the target. In case of rectangular magnetron cathodes, the substrates shall be coated over a range along the target, said range being as large as possible. Typically, the substrates are moved during the coating process such that they pass the target at at least approximately constant speed. This is accomplished, e.g., either in conjunction with a linear movement, or, in particular if the substrates are on a drum, in a convex or concave movement. Because of such a movement, a more or less homogeneous coating with respect to the direction of the movement of the substrate is achieved on the substrates.
Usually, a uniformity of the coating thickness on the substrates as good as possible shall be achieved, in particular in the direction of the movement of the substrate and perpendicularly thereto. In special cases, a specific, pre-determined thickness profile perpendicularly to the direction of the movement of the substrate may be desired. Further below, the invention will be discussed by means of the often-occurring case according to which a uniformity (of the coating thickness) as good as possible shall be achieved. The generalization for the case of a general specific, predetermined coating thickness profile is immediately clear to the person skilled in the art.
A good uniformity usually necessitates correction measures in order to compensate for coating thickness deviations present in systems. Different approaches are used, frequently apertures are arranged between target and substrate by means of which a too high flux of material to the substrate is blocked. Other measures make use of the process gas distribution, or of the plasma discharge via the anode configuration. These measures have in common that the correction measures utilize parts located within the process chamber, and therefore, they can be carried out when the system, in particular the process chamber, is open, i.e. each correction necessitates a ventilation of the chamber resulting in all disadvantages related thereto.
From the above, it is clear that it is desirable to find an alternative method and apparatus for sputter coating of substrates, in particular one that enables an increased target utilization and/or an increased productivity of the coating process.